The effects of chelating agents on radical generation in alkaline peroxide systems, and the relevance to substrate damage Published in: Free Radical Research, 41(5) (Jan 2007), 515-522 EDMUND H. FOWLES1, BRUCE C. GILBERT2, MATTHEW R GILES3 & ADRIAN C. WHITWOOD2 1
EF Chemical Consulting, Chester, CH3 5TH, UK Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK 3 Innospec Ltd, Oil Sites Road, Ellesmere Port, Cheshire, CH65 4EY, UK 2
A spin-trapping EPR technique has been employed to explore the generation of hydroxyl radicals from reactions between a series of first row transition metal ions and aqueous hydrogen peroxide at pH 10, and with a range of chelating agents (EDTA, DTPMP and the readily biodegradable ligands S,S-EDDS and IDS). In the absence of these chelating agents only Cu(II) generates a significant level of hydroxyl radicals; in their presence with Cu(II) EDTA and IDS give similar behaviour whereas EDDS and DTPMP inhibit hydroxyl radical generation. For Fe(II), EDTA, DTPMP, and IDS significantly enhance .OH production under these conditions whereas EDDS does not. Results from model cellulose damage experiments broadly confirm the findings for copper, though experiments with Fe(II) lead to somewhat contrasting results. Our findings are discussed in terms of binding constants and implications for alkaline peroxygen bleaching systems. Keywords: Hydroxyl radical, hydrogen peroxide, chelating agents, cellulose, copper Correspondence: Dr Edmund Fowles, EF Chemical Consulting, 17 Kings Crescent East, Chester, CH3 5TH, UK, Tel: +44-1244-351644, email:
[email protected]
INTRODUCTION Alkaline peroxygen systems play an important part in our daily lives, especially in pulp bleaching, textile bleaching and laundry detergents. The consumption of hydrogen peroxide for pulp bleaching in the US alone is around 250,000 tonnes per annum. The active species in pulp and textile bleaching is the hydroperoxide anion HOO¯, which carries out nucleophilic attack on chromophores such as conjugated carbonyl structures.[1] Household laundry detergents typically contain a bleach activator such as TAED (tetra-acetyl ethylenediamine); peracetic acid is believed to
be the active species at the normal wash-temperatures of around 40 °C, but the hydroperoxide anion is also present. Alkaline hydrogen peroxide is not stable for any length of time so commercially it is supplied at acid pH or as a solid peroxygen-containing compound, such as sodium percarbonate. Once made alkaline it rapidly decomposes to oxygen and water, and trace levels of transition metals are believed to catalyse this decomposition.[2] In order to reduce the loss of hydrogen peroxide, metal-chelating agents are used. EDTA (1) and DTPMP (2) were chosen for this study because they are the most important members of the aminocarboxylate and phosphonate group of chelating agents, respectively, in terms of volumes sold. There are environmental concerns over using traditional chelating agents because of their poor biodegradability, so new, more readily biodegradable agents such as EDDS (3) and IDS (4) are now being considered for these applications[3, 4] and these were also added into this study. For transition metals in alkaline peroxide systems the generation of reactive oxygen species may be relevant. The hydroxyl radical in particular is extremely reactive, causing oxidation of most other species it encounters via hydrogen abstraction or addition to unsaturation and is believed to be the main species responsible for degradation of cellulose in pulp bleaching.[5,6] Hydroxyl and hydroperoxyl/superoxide radicals may also contribute toward dye damage during laundry washing, leading to fading or changing of colour. It has been claimed that addition of chelating agents to peroxide systems prevents the generation of radicals by sequestering of transition metals from the system. However, this is not necessarily true; e.g. with Fe2+, addition of chelating agents can greatly accelerate the generation of hydroxyl radicals (see [7] and references therein). The purpose of the work reported here is to compare systematically the variation of hydroxyl radical generation under alkaline hard water conditions with a range of common transition metals and commercial chelating agents, and to investigate any correlation with cellulose damage. Our approach involved firstly the use of a spin-trapping technique in conjunction with EPR spectroscopy: it was intended to utilise the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO 5) and its reaction with .OH to give the well-known and long-lived adduct (6), detectable by its
characteristic spectrum. Similarly, .OOH (or O2-.) gives the adduct (7). The appearance and intensity of the signals were to be noted as a function of added metals and chelating agents. We also designed a model experiment involving the gravimetric assessment of damage to cellulose. MATERIALS AND METHODS Chemicals All solutions were made up with demineralised water. Calcium and transition metal salts were all chlorides and were laboratory reagent grade from Fisher Scientific or Acros Organics. Hydrogen peroxide was 100-volume laboratory reagent grade from Fisher Scientific. Ethylenediamine tetra-acetic acid, tetra-sodium salt (EDTA, 1) was from BASF under the trade name Trilon® B Liquid. Diethylenetriamine pentamethylphosphonic acid, sodium salt (DTPMP, 2) was from Solutia Inc under the trade name Dequest® 2066. Ethylenediamine disuccinic acid, tri-sodium salt (EDDS, 3) was from Innospec under the trade name Octaquest® E30.. Sodium iminodisuccinate (IDS, 4) was from Bayer under the trade name Baypure® CX-100. DMPO (5) was from Sigma-Aldrich. OH O
O
O
O
HO P OH
HO P OH
OH
N
N
N
HO O
HO P OH
O
O
OH EDTA (1)
O
OH
N N HO P OH HO P OH O O
DTPMP (2)
O
OH O
NH
O
HO
N
OH
N HO HO
O
HO
EDDS (3)
O
OH O
O
IDS (4)
EPR spin trapping: comparison of signal intensities The method employed the spin-trap (DMPO, 5) to trap short-lived free radicals generated to give relatively stable spin-adducts (t½ of the order of minutes), Reactions Me +
Me
N
H
+
.OH
Me Me
H N
O
O·
(5)
(6)
Me +
Me
N
H
+
.OOH
Me Me
OH
(Reaction 1)
H N
O
O·
(5)
(7)
OOH
(Reaction 2)
1 and 2. The DMPO-OH radical adduct (6) has a characteristic EPR spectrum with aN=aH=14.85G. The hydroperoxyl adduct (7) has aN=13.9G and aH=10.85G. EPR spectra were recorded on Bruker ESP-300E spectrometer equipped with an X-band microwave bridge and TE102 cavity. The solution to be analysed was placed in an quartz aqueous flat-cell which was then put into the cavity of the spectrometer. The spectrum sweep was started 2 minutes after mixing of solutions. Typical spectrometer conditions were centre-field 348.0 mT, sweep width 10 mT, modulation amplitude 0.1 mT, microwave frequency ~9.77 GHz, microwave power 10 mW, time constant 160 ms, scan time 167 s. Relative intensities were estimated from peak heights in the first derivative spectra. Typical concentrations were as follows: Solution 1: Transition metal (150 mg/kg, ~ 2.6 mM,) with optional chelating agent (5 mM), (dosed in using a 0.25M stock solution) and pH adjusted to 10 with NaOH immediately prior to addition to reaction mixture. In the case of Fe(II) the solution was sparged with nitrogen before and during the pH adjustment. Solution 2: Hydrogen peroxide (30 mM) containing calcium (7.5 mM, 300 mg/kg), adjusted to pH 10 with NaOH on the morning of the experiment. It was separately established that decomposition of this solution is 60*
Co(II) no chelator
0.0
4
Cr(III) no chelator
2.0
>60*
Cu(II) no chelator
17.4
40
Fe(II) no chelator
6.1
31
Mn(II) no chelator
0.2
6
Ni(II) no chelator
1.7
22
Cu(II)-EDTA (1:1)
7.5
37
Cu(II)-EDDS (1:1)
3.7
35
Cu(II)-DTPMP (1:1)
2.4
13
Cu(II)-IDS (1:1)
12.0
36
Fe(II)-EDTA (1:1)
4.0
18
Fe(II)-EDDS (1:1)
5.1
15
Fe(II)-DTPMP (1:1)
3.3
14
Fe(II)-IDS (1:1)
4.7
18
* concentration of hydrogen peroxide after 1 hour in these 2 experiments was 3.3%.
Table 4. Effect of transition metal and calcium on peroxide decomposition at pH10, 40°C, in the absence of chelating agents Transition metal &
Calcium
% decomposition
concentration (mg/kg)
concentration (mg/kg) after 60 minutes
None
None
5
Fe(II) 4
None
60
Fe(II) 4
40
18
Fe(II) 4
200
15
Cu(II) 5
None
12
Cu(II) 5
100
4
Mn(II) 5
none
43
Mn(II) 5
100
72
